The invention relates to an exhaust gas after-treatment unit for an internal combustion engine, particularly for a motor vehicle, a drive mechanism with such an exhaust gas after-treatment unit and a procedure for using such a drive mechanism.
Exhaust gas after-treatment units for internal combustion engines, especially for motor vehicles, have long been known from the general prior art and particularly from serial vehicle manufacturing. Such an exhaust gas after-treatment unit is used, for example, in a drive mechanism which comprises the exhaust gas after-treatment unit and an internal combustion engine. The drive system is, for example, part of a motor vehicle which is drivable by means of the drive mechanism, particularly by means of the internal combustion engine. The internal combustion engine is a combustion engine.
The internal combustion engine has at least one combustion chamber in the form of a cylinder into which fuel, particularly liquid fuel, and air are fed during the internal combustion engine's fired operation. This creates a fuel air mixture in the combustion chamber which is burned. This results in exhaust gas from the internal combustion engine, wherein the exhaust gas can flow out from at least one internal combustion engine outlet and, therefore, out of the internal combustion engine itself.
By means of an exhaust pipe, for example, the exhaust gas is directed toward the exhaust gas after-treatment unit so that the internal combustion engine exhaust gas can be after-treated using the exhaust gas after-treatment unit. To this end, the exhaust gas after-treatment unit comprises at least one SCR catalytic converter through which the internal combustion engine exhaust gas can flow, by means of which a selective catalytic reduction (SCR) is brought about and supported. This means that the SCR catalytic converter catalyzes the SCR. By means of this selective catalytic reduction, the nitrogen oxide (NOx) contained within the exhaust gas is reduced, meaning that it is at least partially removed from the exhaust gas. In the course of the SCR, the nitrogen oxide contained within the exhaust gas reacts particularly with elements of a reduction agent which is introduced to the exhaust gas or with elements which form from the reduction agent to become nitrogen and water. Without limitation of generality, it is assumed hereafter that the reduction agent deployed within the framework of the present invention is an aqueous urea solution. The ammonia (NH3) which is effective in the SCR for the reduction of nitrogen oxide is created from the aqueous urea solution.
The exhaust gas after-treatment unit further comprises at least one particle filter, through which the exhaust gas can flow, for retaining the exhaust gas soot particles. The exhaust gas is filtered by means of the particle filter so that at least some of the soot particles are filtered from the exhaust gas by means of the particle filter. If the internal combustion engine comprises a diesel engine, for example, the particle filter is usually described as a diesel particle filter (DPF).
The objective of the present invention is to further develop an exhaust gas after-treatment unit of the type already stated which allows for especially favorable operation in terms of exhaust emissions to be realized.
To further develop an exhaust gas after-treatment unit, which makes particularly favorable operation in terms of exhaust emissions feasible, it is envisaged according to the invention that the particle filter which is located downstream from the first SCR catalytic converter in the direction of exhaust gas flow through the exhaust gas after-treatment unit is equipped with a heavy metal and precious metal free catalyzing coating which oxidizes the soot particles held back by the particle filter, wherein there is a second SCR catalytic converter through which the exhaust gas can flow downstream from the particle filter. The heavy metal and precious metal free particle filter coating in the exhaust gas after-treatment unit according to the invention benefits from having no environmentally damaging heavy metals and no other toxic or environmentally damaging materials.
In an embodiment of the invention, the heavy metal and precious metal free particle filter coating contains alkaline and/or alkaline-earth compounds. More preferably, the heavy metal and precious metal free particle filter coating possesses alkaline metal silicate, wherein finely distributed alkaline metals in a silicate structure, especially potassium, are incorporated as active catalytic coating components. Particle filters with a coating according to the embodiment of the invention can catalyze beneficial solid-state reactions with soot particles. Coatings of the particle filter according to the embodiment of the invention can be applied to different substrates such as SIC or Cordierit, for example. Coating the particle filter according to the embodiment of the invention allows for nitrogen dioxide (NO2) based particle filter regeneration, even with small quantities of nitrogen dioxide and/or already low temperatures, as the reaction of the soot or soot particles with nitrogen dioxide in the particle filter, which is catalyzed by means of the coating, is a solid-state reaction, which is catalyzed, meaning it is supported or brought about, by the coating. This reaction can take place with a particularly high reaction rate. At the same temperature conditions, the reaction of the soot with nitrogen dioxide even takes place with smaller quantities of nitrogen dioxide and with higher reaction rates can be observed taking place in a particle filter with a coating according to the embodiment of the invention compared with a particle filter with a precious metal coating. The oxygen (O2) based soot oxidation is also catalyzed using a coating according to the embodiment of the invention and takes place on such coatings even at considerably lower temperatures than in particle filters with precious metal coatings. Therefore, soot can be oxidized with O2 to carbon dioxide (CO2) and steam (H2O) on a coating according to the embodiment of the invention at these low temperatures even when NO2 is excluded, particularly during the dispensing of the aqueous urea solution.
Particle filter regeneration should be understood as at least some of the soot particles which are retained in the particle filter being removed from the particle filter within the framework of the regeneration. With increasing operation times and, therefore, with increasing numbers of exhaust gas soot particles being retained, increasing numbers of soot particles are being added to the particle filter. This addition is also known as particle filter loading. Within the framework of a regeneration, the particle filter loading is at least reduced in that the soot particles are oxidized. This means that the particle filter is, for example, oxidized with NO2 or freely burned with O2 within the framework of the regeneration. The particle filter coating has the role of catalyzing the soot particle oxidation and a coating of the particle filter with alkaline and/or alkaline-earth compounds facilitates an NO2 based particle filter regeneration for significantly smaller quantities of NO2 and with a higher reaction rate than when compared to coating the particle filters with catalytic coatings containing precious metals.
It was surprisingly found that the particle filter coating with alkaline and/or alkaline-earth compounds catalyzes the particle filter regeneration with the help of NO2 particularly well, so that such a regeneration based on NO2 leads to a sufficient soot combustion rate even with low initial concentrations of NO2, such as the internal combustion engine's NO2 raw emissions, and that it is not essential to continually carry out NO2 based regeneration in particle filters with such a coating, rather that a regeneration which is performed intermittently is sufficient. Regeneration with the help of NO2 is referred to as passive regeneration.
Because O2 based particle filter regeneration takes place at significantly lower temperatures in particle filters with alkaline and/or alkaline-earth compound coatings than in particle filters with precious metal coatings, the O2 based regeneration will support the NO2 based regeneration even at temperatures from around 300 to 350 degrees Celsius in particle filters with a coating with alkaline and/or alkaline-earth compounds. The O2 based soot regeneration can also sometimes replace the NO2 based regeneration within a temperature window of 300 to 350 degrees Celsius if the NO2 based regeneration is restricted or fails completely due to low NO2 concentrations, as is the case when the total amount of NO2 present in the exhaust gas is consumed in the SCR reaction in the preceding first SCR catalytic converter. In particle filters with, known precious metal coatings, the O2 based soot oxidation rates within a temperature range of around 300 to 350 degrees Celsius is considerably lower than in particle filters with a coating with alkaline and/or alkaline-earth compounds and, therefore, do not contribute to soot combustion.
Due to the fact that O2 based particle filter generation takes place even within a temperature range of around 300 to 350 degrees Celsius in particle filters with an alkaline and/or alkaline-earth compound coating, an O2 based particle filter regeneration can be used without the disadvantageous undesired damage to the exhaust gas after-treatment elements, which can happen with the high temperatures from O2 based regenerations for traditional precious metal particle filters.
A further characteristic of the coating according to the embodiment of the invention used in the exhaust gas after-treatment unit according to the invention is that the coating and, therefore, the particle filter, does not possess any catalytic activity regarding gas-gas reactions. This means that the chemical reaction which turns nitrogen monoxide (NO) to nitrogen dioxide (NO2) is not catalyzed by this coating. This shortage of catalytic activity regarding gas/gas reactions is highly significant for the exhaust gas after-treatment unit according to the invention as it is because of this that the reduction agent dosing system can be positioned close to the internal combustion engine, for example, right next to the turbo charger. Reduction agent which has possibly been incompletely converted or NH3 which has been desorbed by the first SCR catalytic converter is not oxidized to NO or N2O on the particle filter coating and can be further used in the second SCR catalytic converter for NOx reduction. There is no ammonia-blocking catalytic converter (ABC) necessary after the first SCR catalytic converter and no second reduction agent dosing position required in front of the second SCR catalytic converter. This saves costs and reduces the complexity of the exhaust gas after-treatment unit according to the invention. Furthermore, the particle filter with a coating with alkaline and/or alkaline-earth compounds can be used to improve the urea preparation, particularly to distribute the urea uniformly and to blend it with the exhaust gas.
The invention is particularly based on the conclusion that high nitrogen oxide emissions can generally be generated, particularly following an internal combustion engine start, particularly following a cold start, as well as following a motor vehicle operation in the low load range, particularly following an idle operation, also including coasting mode, during which the internal combustion engine is in its idle operation, as well as following traffic light waiting periods because the catalytic converters and filters for the exhaust gas after-treatment unit cool down in these motor vehicle operation modes and are so cold following these motor vehicle operation modes that the catalytic converters and filters must first be brought up to working temperature in the following start-up processes or acceleration processes, during which very high exhaust emissions are generated.
Exhaust gas after-treatment unit catalytic converter and filter cooling following such internal combustion engine low load operations happens more severely with correspondingly higher exhaust gas emissions for motor vehicles in the form of commercial vehicles or heavy-goods vehicles than with passenger cars because there is a comparatively large interval with an associatively large distance between the internal combustion engine and an exhaust gas after-treatment unit in commercial vehicles or heavy-goods vehicles compared to in passenger cars, whereby higher thermal losses occur in commercial vehicles or heavy-goods vehicles than in passenger cars.
For a conventional exhaust gas after-treatment unit, the introduction of reduction agent into the exhaust gas is switched off at the above-mentioned operating conditions, that means at and for a heating phase following a start, particularly a cold start, and also for a heating phase following a low load operation, as the exhaust gas has a very low temperature at these operating conditions. The introduction of reduction agent is switched off here so that the reduction agent does not crystallize. The introduction of the reduction agent is normally only switched on or implemented when an SCR catalytic converter, in which the reaction agent should be implemented, has a temperature which is higher than 180 degrees Celsius. Switching off the introduction of the reduction agent results in high nitrogen oxide emissions during the specified operating conditions if no corresponding countermeasures are met.
By using the particle filter with the coating according to the embodiment of the invention, as well as using the second SCR catalytic converter, excessive nitrogen oxide emissions can also be omitted for the described start-up or acceleration procedures following an internal combustion engine initial phase or a low load operation. Through mounting the first SCR catalytic converter at the very front of the exhaust gas after-treatment unit according to the invention in front of the particle filter, this first SCR catalytic converter is heated more quickly following an internal combustion engine cold start or following a low load operation as an additional temperature decrease before entering the SCR catalytic converter, which occurs in particle filters in conventional exhaust gas after-treatment units due a high thermal capacity, is omitted so that reduction agents can be dosed comparatively more quickly following an internal combustion engine cold start or a low load operation in the exhaust gas after-treatment unit according to this invention and nitrogen oxide can therefore be converted more quickly following an internal combustion engine cold start or a low load operation. In this way, nitrogen oxide emissions can be further reduced with an exhaust gas after-treatment unit according to the invention. Furthermore, in comparison to conventional exhaust gas after-treatment units on an oxidizing catalytic converter, particularly a DOC, it is possible to avoid this, which would cause a further temperature reduction because of its thermal capacity, so that an even more emission efficient operation can be obtained with the exhaust gas after-treatment unit according to the invention. Additionally, an especially cost and weight efficient exhaust gas after-treatment unit is possible by the omission of an oxidizing catalytic converter. A DOC application leads to high NO2 percentages for internal combustion engine idle operations. The underlying idea for the invention is to use the particle filter coating with alkaline and/or alkaline-earth compounds to use the NO2 proportion in the internal combustion engine exhaust gas, particularly following a cold start or following operation with low loads and speeds, wherein a very good cold start and emission procedure can be realized by means of the exhaust gas after-treatment unit according to the invention. Furthermore NO2 secondary emissions are kept low, particularly during urban operation, particularly through NO2 proportions in which nitrogen oxide is less than or equal to 50 percent. Furthermore, a particularly quick, O2 based soot combustion can be realized at low exhaust gas temperatures. In particular, a quick O2 soot combustion at 420 to 450 degrees Celsius, instead of at 600 degrees Celsius as is envisaged for conventional exhaust gas after-treatment units, is possible, whereby the exhaust gas after-treatment unit's thermal aging, particularly for the SCR catalytic converter and the particle filter, can be kept low.
In order to realize an operation which is particularly advantageous in terms of emissions, it is intended in the beneficial embodiment of the invention that a dosing unit is located upstream from the first SCR catalytic converter, by means of which a reduction agent for denoxing the exhaust gas can be introduced to the exhaust gas. This means that, by means of the dosing unit, the reduction agent is introduced to the exhaust gas at a specific place, wherein this place is located upstream from the first SCR catalytic converter relative to the direction of exhaust gas flow through the exhaust gas after-treatment unit. The previously described, at least partial removal of nitrogen oxide from the exhaust gas is understood as denoxing the exhaust gas. Within the framework of the selective catalytic reduction, an aqueous urea solution, ammonia, is created from the reduction agent which can react with the nitrogen oxide contained within the exhaust gas to create nitrogen and water. Due to the fact that the NH3 which is formed from the aqueous urea solution and remains in the exhaust gas is not oxidized on the coating with alkaline and/or alkaline-earth compounds on the particle filter following the first SCR catalytic converter, because coatings with alkaline and/or alkaline-earth compounds do not oxidize gaseous components, a further dosing unit before the second SCR catalytic converter in the exhaust gas after-treatment unit according to the invention can be beneficially omitted so that, beneficially, only one distribution unit is required in the exhaust gas after-treatment unit according to the invention.
By using the particle filter with the specified particle filter coating with alkaline and/or alkaline-earth compounds, it is also conceivable that a hydrocarbon dosing unit, a so-called HC doser, to introduce unconsummated hydrocarbons to the exhaust gas can be omitted and that the costs for the exhaust gas after-treatment unit according to the invention can be further reduced. Furthermore, it is also conceivable to forgo an exhaust gas recirculation (EGR) so that the exhaust gas after-treatment unit costs can be kept especially low.
In an embodiment of the invention, a control and regulating system for the exhaust gas after-treatment unit is included in a drive mechanism which, periodically and/or under given operating conditions, causes an injection or switching off of reduction agent for a predefined time frame. In a beneficial embodiment of the invention, the control and regulating system is executed as a switch-off device, by means of which an injection of reduction agent into the exhaust gas as effected by the dosing unit can be deactivated. One of the invention's underlying ideas is to temporarily interrupt the injection of the reduction agent, which is particularly an aqueous urea solution, in order to carry out a passive regeneration of the particle filter based on NO2. The passive regeneration is preferably not carried out continuously, rather in discontinuous or periodic batches.
It has also proved itself to be especially beneficial when the first SCR catalytic converter is the first exhaust gas after-treatment element through which the exhaust gas passes downstream from the internal combustion engine. In other words, the first SCR catalytic converter is the first exhaust gas after-treatment element through which the exhaust gas from the internal combustion engine passes after the exhaust gas has exited the internal combustion engine so that, relative to the direction of exhaust gas flow from the internal combustion engine to the first SCR catalytic converter, there is no exhaust gas after-treatment element to after-treat the internal combustion engine exhaust gas between the first SCR catalytic converter and the first SCR catalytic converter. In comparison to conventional exhaust gas after-treatment units, it is therefore possible, for example, to omit an oxidizing catalytic converter and, instead of this, to place the first SCR catalytic converter as the first exhaust gas after-treatment element through which the exhaust gas flows, wherein a particularly high first SCR catalytic converter NOx conversion can be realized due to high exhaust gas temperatures, particularly under the aforementioned operating conditions.
This embodiment is particularly based on the knowledge that the exhaust gas essentially cools by flowing through exhaust gas after-treatment elements, as such exhaust gas after-treatment elements always have a high thermal capacity. The cooling of the exhaust gas can only be subordinately traced back to having to a travel a long distance. This means that the omission or dropping of a particle filter and/or an oxidizing catalytic converter, particularly a DOC, in front of the first SCR catalytic converter in the direction of emission gas flow does not only result in cost savings, rather the omission of a particle filter and/or an oxidation catalytic converter is also beneficial in as far as excessive exhaust gas cooling caused by a particle filter and/or an oxidation catalytic converter can be omitted because the exhaust gas does not need to pass through a particle filter and/or an oxidation catalytic converter on its way from the internal combustion engine to the first SCR catalytic converter. The exhaust gas, therefore, benefits from having a particularly high temperature when reaching the first SCR catalytic converter so that high first SCR catalytic converter NOx conversion rates can be realized. It should be noted, that a temperature decrease due to flowing through a particle filter is generally considerably greater than a temperature decrease due to a DOC because a particle filter material has a higher thermal capacity than a DOC material and, additionally, a particle filter is generally designed to be larger. Beneficially, the exhaust gas after-treatment unit according to the embodiment of the invention can be stored particularly in a standard exhaust gas box, a so-called One Box, of a current mass-produced heavy goods or commercial vehicle whilst the first SCR catalytic converter can be inserted in the omitted oxidizing catalytic converter's place in the exhaust gas after-treatment unit, in front of a particle filter and a SCR/ASC catalytic converter in the direction of flow. The exhaust gas after-treatment unit according to this embodiment of the invention can be beneficially inserted into current mass-produced commercial vehicles without taking up extra space.
Beneficially, no further exhaust gas box is required for an exhaust gas after-treatment unit according to this embodiment of the invention, rather it is placed in the DOC so that the exhaust gas after-treatment unit according to the invention can be presented in a current mass produced commercial vehicle without taking up any extra space.
The high exhaust gas temperatures, as seen in the first SCR catalytic converter of the exhaust gas after-treatment unit according to the invention, encourages a good urea stock preparation so that the exhaust gas can be particularly well denoxed by means of the reduction agent. Therefore, a particularly good urea stock preparation upstream from or directly following an exhaust gas turbo charger, particularly following an exhaust gas turbo charger, is possible. A urea dosage in front of the exhaust gas turbo charger in the direction of exhaust gas flow is also conceivable.
Beneficially, the particle filter is the second exhaust gas after-treatment element in the exhaust gas after-treatment unit according to the invention through which the exhaust gas passes after the exhaust gas has exited the internal combustion engine so that there is beneficially no exhaust gas after-treatment element to after-treat the internal combustion engine exhaust gas between the particle filter and the first SCR catalytic converter.
It has proved itself additionally beneficial if the second SCR catalytic converter is the third exhaust gas after-treatment element through which the exhaust gas passes after the exhaust gas has exited the internal combustion engine so that there is beneficially no exhaust gas after-treatment element to after-treat the internal combustion engine exhaust gas between the second SCR catalytic converter and the particle filter.
In an embodiment of the invention, the first SCR catalytic converter has a smaller ammonia storage capacity than the second SCR catalytic converter. Additionally, in a beneficial embodiment of the invention, at least one of the SCR catalytic converters in the exhaust gas after-treatment unit according to the invention, particularly the first SCR catalytic converter, is configured as a Vanadium SCR catalytic converter, particularly with a V2O5 coating. V2O5 has a similar effect to a DOC and catalyzes an oxidation of NO to NO2 so that a particle filter passive regeneration is increased, V2O5 also has a beneficially lower laughing gas selectivity (N2O selectivity) for higher NO2 presences. V2O5 also benefits from having a smaller NH3 storage capacity so that the Vanadium SCR catalytic converter can fill more quickly with NH3 after its NH3 empty run and a quicker nitrogen oxide turnover can, therefore, take place. It is particularly beneficial to implement the second SCR catalytic converter as a copper SCR (Cu SCR). A Cu SCR benefits from having good low temperature activity, good nitrogen oxide reduction rates even with small NO2/NOx ratios and a high NH3 storage capacity, particularly a higher NH3 storage capacity than a vanadium SCR catalytic converter, so that the nitrogen oxide reduction with NH3 in the second SCR catalytic converter implemented as a Cu SCR with the NH3 stored in the second SCR catalytic converter can take place particularly beneficially for an exhaust gas after-treatment unit according to the invention in a particle filter NO2 regeneration operation, with a first SCR catalytic converter designed as a vanadium SCR and a second SCR catalytic converter designed as a Cu SCR.
A further embodiment distinguishes itself in that the first SCR catalytic converter has a first volume through which the exhaust gas can flow and the second SCR catalytic converter has a second volume through which the exhaust gas can flow, wherein the first volume is smaller than the second volume. This enables a particularly efficient operation in terms of emissions to be realized. This embodiment of the invention also benefits from a nitrogen oxide reduction with NH3 taking place in the second SCR catalytic converter with the NH3 which is still stored in the second SCR catalytic converter after a reduction agent injection deactivation.
In one embodiment of the invention, an ammonia slip catalyst (ASC) through which the exhaust gas can flow is located downstream from the second SCR catalytic converter. Such an ASC has the objective of turning a possible reduction agent excess or ammonia into nitrogen and water so that an especially efficient operation in terms of emissions can be realized. Furthermore, unpleasant odors can also be effectively avoided. It was found that an especially efficient operation in terms of emissions for the exhaust gas after-treatment unit according to the invention can be realized if the first SCR catalytic converter's volume has a ratio to a total volume which is composed of the volumes of the second SCR catalytic converter and the volume of the ammonia slip catalyst of approximately 0.3 to 0.8.
For a further beneficial embodiment of the invention there is a dosing unit located upstream from the particle filter, by means of which unconsummated hydrocarbons can be introduced to the exhaust gas. This means that, by using the dosing unit to introduce unconsummated hydrocarbons (HC) to the exhaust gas, the unconsummated hydrocarbons are introduced to the exhaust gas at a specific place, wherein this place is located upstream from the particle filter. By introducing unconsummated hydrocarbons (HC) to the exhaust gas, the exhaust gas temperature can be raised especially effectively and according to requirement via the exothermic oxidation of HC on the first SCR catalytic converter, so that an especially efficient operation in terms of emissions can be represented. Furthermore, it is possible to use this to support or activate the active particle filter regeneration based on O2, wherein this can be carried out at comparatively low temperatures. Surprisingly, it is possible to carry out both active and passive regeneration under particularly beneficial conditions by means of the particle filter coating with alkaline and/or alkaline-earth compounds and to therefore cause an effective soot reduction in the particle filter.
The dosing unit to introduce the unconsummated hydrocarbons to the exhaust gas is preferably located particularly close to the internal combustion engine, meaning it is close to the combustion engine in order to introduce the unconsummated hydrocarbons to the exhaust gas, for example, when this still has a relatively high temperature.
The invention also includes a drive mechanism for a motor vehicle, particularly a commercial vehicle, with an internal combustion engine and with an exhaust gas after-treatment unit according to the invention. Beneficial embodiments of the exhaust gas after-treatment unit according to the invention should be seen as beneficial embodiments of the drive mechanism according to the invention and vice-versa.
Furthermore, a procedure for operating a drive mechanism according to the invention also belongs to the invention. Beneficial embodiments of the drive mechanism and the exhaust gas after-treatment unit according to the invention should be seen as beneficial embodiments of the procedure according to the invention and vice-versa.
It has proved itself particularly beneficial within the framework of the procedure according to the invention if an exhaust gas temperature raise is effected by at least one measure within the combustion engine relative to the internal combustion engine. As the internal combustion engine is also described as a combustion engine, the measure is also described as a measure within the combustion engine or as an intervention within the combustion engine. The exhaust gas temperature can be purposefully increased using such an intervention within the combustion engine so that, for example, particularly effective, active O2 based particle filter regeneration can be carried out. An intervention within the combustion engine involves, for example, reducing the air mass flow rate which is carried out in at least one of the internal combustion engine's combustion chambers, particularly in the form of a cylinder. The air mass flow rate is reduced by throttling, for example. Furthermore, the intervention within the combustion engine can also include a late adjustment to the main fuel injection. Alternatively or additionally, it is conceivable that the intervention within the combustion engine comprises the implementation of downstream after-injections which only partially reburn in the combustion chamber or in the combustion engine.
Ultimately, it has also proved itself especially beneficial if the introduction of reduction agent to the exhaust gas within the framework of the procedure according to the invention is periodically deactivated or stopped under specific operating conditions for a predefined period of time. In other words, the introduction of reduction agent is designed to be periodically deactivated under certain operating conditions for a predefined period of time. Deactivating the introduction of reduction agent into the exhaust gas is particularly beneficial for carrying out passive NO2 based regeneration so that the NO2 which is required for the NO2 based particle filter regeneration is not broken down in the first SCR catalytic converter of the exhaust gas after-treatment unit according to the invention.
For the NO2 based particle filter regeneration, it is possible in the exhaust gas after-treatment unit according to the invention to interrupt the reduction agent infeeding, meaning the introduction of the reduction agent, for a set period of time because the second SCR catalytic converter has a certain NH3 storage capacity or storage space. It is understood under storage capacity or storage space that a certain amount of ammonia (NH3) can be stored in the second SCR catalytic converter. The second SCR catalytic converter's ultimate NH3 storage capacity is larger in the exhaust gas after-treatment unit according to the invention than for the first SCR catalytic converter. This is designed to realize a particularly beneficial exhaust gas after-treatment unit operation. The SCR catalytic converters' varying NH3 storage capacities can be realized, for example, in such a way that the SCR catalytic converters—as described before—are dimensioned differently, wherein the first. SCR catalytic converter, or rather its first volume, is smaller than the second SCR catalytic converter, or rather its second volume, and/or by designing a first and second SCRs with specific, different NH3 storage capacities. For example, the NH3 which is stored in the second SCR catalytic converter for the time period during which the introduction of reduction agent into the exhaust gas for an NO2 based particle filter regeneration is deactivated suffices for the nitrogen oxide (NOx) leaving the particle filter in the second SCR catalytic converter to dissipate. An especially efficient operation in terms of emissions can therefore be realized if the introduction of reduction agent into the exhaust gas stops on short notice for a predefined period of time.
It is recognizable that the particle filter coating with alkaline and/or alkaline-earth compounds distinguishes itself in that the coating catalyzes soot oxidation selectively. In contrast to conventional, precious metal particle filter coatings, particularly diesel particle filters, the particle filter coating with alkaline and/or alkaline-earth compounds for the exhaust gas after-treatment unit according to the invention has no catalytic reactivity for gas to gas reactions, under which reactions from NO to NO2, CO to CO2, HC to CO2+H2O and NH3 oxidation fall. By means of the exhaust gas after-treatment unit according to the invention, the drive mechanism according to the invention and the procedure according to the invention, secondary emissions such as NO2 und N2O, particularly under real driving conditions, can be kept low by simultaneously realizing a beneficial and efficient particle filter regeneration.
Further advantages, characteristics and details of the invention can be seen from the following description of a preferred exemplary embodiment and with reference to the drawings. The characteristics and combinations of characteristics stated above in the description and the characteristics and combinations of characteristics stated below in the description of the drawings and/or shown in the drawings alone can be used not only in the specified combination in each case, but also in other combinations or in isolation without departing from the scope of the invention.